Technical Field
The present disclosure generally relates to implementations of transistors for use in high-speed, high-frequency integrated circuits and, in particular, to vacuum channel transistors.
Description of the Related Art
Vacuum channel transistors have been proposed as a solution to overcome performance limitations associated with conventional planar silicon metal-oxide-semiconductor field effect transistors (MOSFETs), for example, in U.S. Pat. No. 6,437,360 to Cho et al., and U.S. Pat. No. 8,159,119 to Kim et al. FIG. 1A provides a comparison of the basic structure of a conventional MOSFET 70 with that of an existing vacuum channel transistor 72 designed by NASA in collaboration with the National Nanofabrication Center of Korea [Applied Physics Letters, volume 100, published May 23, 2012]. The conventional MOSFET 70 shown on the left side of FIG. 1A includes a source 76, a drain 78, a gate 80, a channel 82, and a gate dielectric 84 formed on a semiconductor substrate. The conventional MOSFET 70 operates as follows: the source 76 and drain 78 are doped with positive or negative ions to provide reservoirs of charge. In response to a voltage applied to the gate 80, a current is induced to flow in the channel 82, thereby coupling the source 76 and the drain 78. The channel of the conventional MOSFET 70 lies between the doped source and drain regions and thus is made of the semiconductor material, typically silicon.
As current flows between the source 76 and the drain 78, the motion of electrons through the silicon crystal is impeded by the presence of silicon atoms and impurities in the crystal. In the conventional MOSFET 70, electrons can also experience scattering from acoustic phonons associated with the crystal lattice, among other sources. Consequently, increasing electron mobility has been a topic of great interest and activity in the semiconductor field for decades. Performance improvements for semiconductor channel devices have relied on influencing mechanical properties, e.g., strain, of the silicon lattice, for example, by introducing adjacent layers of different materials or by replacing portions of the silicon with epitaxially grown, and/or doped, crystalline material.
The vacuum channel transistor 72, like the conventional MOSFET 70, has a source 86, a drain 88, a gate 90, an air channel 92, and a gate dielectric 94 formed on a semiconductor substrate. However, the vacuum channel transistor 72 offers a different approach from that of the conventional MOSFET 70 in that the channel 92 does not include crystalline material. The structure of the vacuum channel transistor 72 is upside down, such that the gate is positioned below the source and drain terminals, and the air channel 92 is an open region between the source and drain. Furthermore, the source and drain 86, 88, respectively, are shaped with points to enhance electric fields during operation of the vacuum channel transistor 72. When the gate 90 is energized, a current flows between the source 86 and the drain 88 by thermionic emission, or “arcing.” If the points of the source and drain 86, 88 are spaced closely enough to one another, the voltage required to cause thermionic emission that activates the device may be relatively small. The trajectory of emitted electrons may then be shorter than the distance between air molecules, permitting the electrons to travel ballistically through the air channel without being impeded by collisions. Such ballistic motion is effectively the same as that which would occur if the air channel 92 was evacuated. Thus, a vacuum channel transistor need not actually contain a vacuum, but may be filled with air, and the electrons will still travel substantially as fast as they would in a vacuum. Consequently, the velocity of electrons in the vacuum channel transistor 72 can be up to 1000 times faster than the velocity of electrons traversing a semiconductor channel, causing the vacuum channel transistor 72 to switch on and off fast enough to operate at frequencies in the range of 100 GHz to 1 Terahertz as illustrated in FIG. 1B [IEEE Spectrum, July 2014, p. 35]. Such a device has many potential applications, for example, in high-speed telecommunications.